1. Technical Field
This invention relates generally to the field of cathodic protection systems for steel-reinforced concrete structures, and is particularly concerned with the performance of cathodic protection systems utilizing thermally sprayed zinc, zinc alloy, or aluminum anodes.
2. Description of the Prior Art
The problems associated with corrosion-induced deterioration of reinforced concrete structures are now well understood. Steel reinforcement has generally performed well over the years in concrete structures such as bridges, buildings, parking structures, piers, and wharves, since the alkaline environment of concrete causes the surface of the steel to "passivate" such that it does not corrode. Unfortunately, since concrete is inherently somewhat porous, exposure to salt results in the concrete over a number of years becoming contaminated with chloride ions. Salt is commonly introduced to the concrete in the form of seawater, set accelerators or deicing salt.
When the chloride contamination reaches the level of the reinforcing steel, it destroys the ability of the concrete to keep the steel in a passive, or non-corrosive state. It has been determined that a chloride concentration of 0.6 Kg per cubic meter of concrete is a critical value above which corrosion of steel can occur. The products of corrosion of the steel occupy 2.5 to 4 times the volume of the original steel, and this expansion exerts a tremendous tensile force on the surrounding concrete. When this tensile force exceeds the tensile strength of the concrete, cracking and delaminations develop. With continued corrosion, freezing and thawing, and traffic pounding, the utility or the integrity of the structure is finally compromised and repair or replacement becomes necessary. Reinforced concrete structures continue to deteriorate at an alarming rate today. In a recent report to Congress, the Federal Highway Administration reported that of the nation's 577,000 bridges, 226,000 (39% of the total) were classified as deficient, and that 134,000 (23% of the total) were classified as structurally deficient. Structurally deficient bridges are those that are closed, restricted to light vehicles only, or that require immediate rehabilitation to remain open. The damage on most of these bridges is caused by corrosion of reinforcing steel. The United States Department of Transportation has estimated that $90.9 billion will be needed to replace or repair the damage on these existing bridges.
Many solutions to this problem have been proposed, including higher quality concrete, improved construction practices, increased concrete cover over the reinforcing steel, specialty concretes, corrosion inhibiting admixtures, surface sealers, and electrochemical techniques such as cathodic protection and chloride removal. Of these techniques, only cathodic protection is capable of controlling corrosion of reinforcing steel over an extended period of time without complete removal of the salt contaminated concrete.
Cathodic protection reduces or eliminates corrosion of the steel by making it the cathode of an electrochemical cell. This results in cathodic polarization of the steel, which tends to suppress oxidation reactions (such as corrosion) in favor of reduction reactions (such as oxygen reduction). Cathodic protection was first applied to a reinforced concrete bridge deck in 1973. Since then, understanding and techniques have improved, and today cathodic protection has been applied to over one million square meters of concrete structure worldwide. Anodes, in particular, have been the subject of much attention, and several types of anodes have evolved for specific circumstances and different types of structures.
One type of anode which has recently been utilized for cathodic protection of reinforced concrete structures is thermally-sprayed zinc or zinc alloy. In this case thermal energy is used to convert a zinc or zinc alloy to its molten or semi-molten state, which is then deposited onto a prepared substrate. The zinc or zinc alloy may originally be in the form of powder, wire or rod. Thermal energy is generated by using combustible gases or an electric arc. As the zinc or zinc alloy is heated, it changes to a molten or plastic state, and is then accelerated by a compressed gas to the substrate surface. The particles strike the surface where they conform and adhere to the irregularities of the prepared surface and to each other. As the sprayed particles continue to impinge upon the substrate, they cool and build up, particle by particle, thus forming a coating. It has been determined in a recent survey that zinc anodes have been utilized for cathodic protection on 50,000 square meters of reinforced concrete structures.
This zinc or zinc alloy coating may then be used as an anode to supply current for the cathodic protection process. Such anodes may be used for either sacrificial or impressed current cathodic protection systems. Sacrificial cathodic protection systems are simpler and less expensive to install and maintain than impressed current systems, first because an ancillary power supply is not needed, and also because intentional shorts between the anode and steel are not detrimental to the system. For sacrificial systems a direct electrical connection is made between the anode and the reinforcing steel, and current flows spontaneously since the electrochemical reactions which cause current flow are thermodynamically favored. The amount of current which flows is uncontrolled, and is dependent mainly on the resistance of the concrete, the geometric relationship between the anode and steel, and the age of the system. The current which flows from sacrificial systems is sometimes insufficient to meet cathodic protection criteria. For this reason, the use of sacrificial systems is usually limited to locations where the concrete is very conductive due to high moisture and chloride content, such as in the seawater splash and tidal zone. Even so, cathodic protection systems utilizing zinc or zinc alloy anodes always experience a current decrease with time. After a few months, or at most, a very few years, current flow will decrease to the point where it is insufficient to meet cathodic protection criteria, at which point the anode will have to be removed and replaced. Removal and subsequent replacement of the anode by thermal spray process involves significant expense.
Where zinc and zinc alloy anodes are used in impressed current systems, a power supply is connected between the anode and the reinforcing steel. The power supply is used to increase the driving force (voltage) between the anode and cathode. In this case, the voltage may be increased so that the current needed for cathodic protection is maintained for a much longer period of time. Even so, after a few years the cathodic protection system voltage may exceed the design maximum of the power supply, usually 24 volts, and the current will thereafter become insufficient to meet cathodic protection criteria. This phenomenon of declining current from zinc and zinc alloy anodes has been a major limitation for the use of zinc and zinc alloy anodes, both for sacrificial and for impressed current cathodic protection systems. The exact cause of this phenomenon is not known, but is generally thought to be related to the build-up of anode corrosion products, such as zinc oxides and hydroxides, at the interface between the anode and the concrete.